1. Field of the Invention
The present invention is generally related to optical component manufacturing and more particularly to protective coatings used in manufacturing optical components.
2. Technical Background
Variable optical attenuators (VOAs), 1xc3x972 switches, and 2xc3x972 switches are non-limiting examples of photonic devices which use a multiclad coupler. In these applications, the coupler requires a protective coating at the taper region to protect the coupler from breakage during the normal handling associated with assembly of the devices as well as during the functioning of the device. In the devices listed above, the coupler is flexed to attenuate the light signal propagating in the device. Any properties of the coating that degrade the optical signal are undesirable. Thus, the application of the coating must not negatively impact the attenuation response of the coupler after it is incorporated into the device.
FIG. 1 depicts an exemplary variable optical attenuator (VOA) 20 which uses a multiclad coupler 22 and a servomotor 26. Coupler 22 includes an input fiber 28 and two output fibers 30 and 32. First output fiber 30 is the output of VOA 20 and second output fiber 32 acts as a xe2x80x9cdead-endxe2x80x9d lead. An optical signal passes from input fiber 28 to either first output 30 or second output 32 through taper region 24 which couples the light signal from fiber 30 to fiber 32. Flexing coupler 22 at taper region 24 by different amounts via servomotor 26 causes more or less of the light signal to be transmitted to the dead-end fiber 32. The amount of flexing controls the attenuation of the signal. Thus, tapered region 24 functions as a commutator.
FIG. 2 depicts an optical step response 10 that was generated by an optical switch having a coupler without a coating in the tapered region. As depicted in FIG. 1, the tapered region is moved between a first unflexed position to a second flexed position, at the time of switching, Tsw. The first position corresponds to a signal transmission state 12 wherein the insertion loss is approximately zero. The second state corresponds to a signal attenuation state 14 wherein the insertion loss is approximately 19.3 dB. Note that the plot of the insertion loss as depicted in FIG. 1a is a square-wave. The insertion loss in both the first state and the second state is substantially constant. This is a desired response. Unfortunately, the coupler represented by FIG. 2 does not have a coating. It is unprotected and susceptible to breakage.
In one approach that has been taken, couplers have been coated with a cationic ultraviolet (UV) curable epoxy system. FIG. 3 depicts the insertion loss response 10 of the switch of FIG. 2 having a coupler that is coated with the cationic UV epoxy. Again, the tapered region is moved between a first unflexed position to a second flexed position, at the time of switching, Tsw. ILswc is the peak insertion loss of the coated coupler at the time of switching (Tsw). ILswc overshoots the insertion loss ILswu of the uncoated coupler in the attenuation state. ILswu is used a reference insertion loss value. Peak insertion loss ILswc is followed by hysteresis 12, which is the decay of the peak insertion loss ILswc to ILswu. ILxcex94sw=¦ILswcxe2x88x92ILswu¦ and represents the absolute value of the difference between the peak insertion loss of the coated coupler at the time of switching and the insertion loss of the uncoated coupler in the second state. As shown in FIG. 3, ILxcex94sw=23 dBxe2x88x9219.3 dB=3.7 dB. This formula is used to accommodate a coating material that generates a peak insertion loss ILswc that undershoots ILswu.
It is useful to measure hysteresis 12 in terms of its decay time TD. The decay time TD is a measure of the time it takes for peak insertion loss ILswc to decay to ILswu. As depicted in FIG. 3, the cationic ultraviolet (UV) curable epoxy system produces transients that have a decay time TD lasting approximately 14 seconds. As depicted, the decay of the transient hysteresis continues for several minutes. In more rigorous terms, TD is defined as TD=T1xe2x88x92Tsw, wherein Tsw is the time at which the coupler is switched from the first state to the second state, and T1 is the time at which peak insertion loss ILswc decays to ILD. ILD=(0.27)ILxcex94sw=(0.27)¦ILswcxe2x88x92ILswu¦, which represents an exponential decay over time.
When the device is commutated from the second position to an unflexed first position at time Tusw, a second hysteresis 16 is generated. The analysis discussed above with respect to hysteresis 12 can be used to analyze hysteresis 16. As depicted, its decay time will also last several minutes. Both hysteresis 12 and hysteresis 16 are undesirable and illustrate the unwanted transients produced by the coating immediately after switch commutation. Another drawback to the cationic ultraviolet (UV) curable coating is that it is colorless. It is difficult to determine that the coating has been applied.
What is needed is a protective coating that does not generate the unwanted optical transients and hysteresis produced by earlier approaches. An optical device is needed that settles into a quiescent state immediately after commutation. Furthermore, the protective coating should include a tinted material. Since clarity is important, the tinted material should allow internal areas in the coupler to be viewed through the coating.
The present invention overcomes the aforementioned disadvantages as well as others. In accordance with the teachings of the present invention, the coating protects optical devices without generating unwanted optical side effects during flexing. The coating adheres readily to the glass of the waveguide component. The coating has a tint so that it can be readily ascertained that the coating has been applied, but also has sufficient clarity so that the internal areas in the component may be viewed. In one embodiment a UV coating cures to a tack free state in air so that a nitrogen blanket is not required during the cure. A solvent based coating such as a lacquer can also be used. The coating also does not degrade when exposed to relatively severe environmental conditions.
One aspect of the present invention is an optical device for directing a light signal. The optical device includes a commutation region movable between a first position corresponding to a signal transmission state, and a second position corresponding to a signal attenuation state. A protective coating is disposed on the commutation region that does not substantially introduce insertion loss transients when the commutation region is moved between the first position and the second position.
In another aspect, the present invention includes a method of directing a light signal in an optical device having a first output, and a commutation region movable between a first position corresponding to a signal transmission state, and a second position corresponding to a signal attenuation state. The method includes the steps of applying a protective coating onto the commutation region. Directing a light signal into the optical device. Moving the commutation region from the first position to the second position to thereby attenuate the light signal in the first output, whereby the protective coating does not substantially produce insertion loss transients in the optical device.
In yet another aspect, the present invention includes a method of fabricating an optical device, the optical device having a commutation region movable between a first position corresponding to a signal transmission state, and a second position corresponding to a signal attenuation state. The method including the steps of providing a coating material. Applying the coating material to the commutation region, wherein the coating material does not substantially produce insertion loss transients when the commutation region is moved between the first position and the second position.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.